The present invention is directed to laminates comprising polyurethanes.
Thermoplastic polyurethanes are used for preparing safety glass and more complex laminates comprising glass and polycarbonate layers, such as those disclosed in U.S. Pat. No. 3,388,032 to Saunders. One problem in manufacturing assymetrical laminates comprising dissimilar outer layers is caused by the different coefficients of thermal expansion of the layers. For example, plastics usually have a coefficient of expansion 5 to 15 times that of glass. Prior art polyurethane formulations require a laminating temperature of 200.degree. F. or more. Three-layered laminates using layers of dissimilar materials can not be manufactured using prior art polyurethane formulations, because the product laminate tends to warp on cooling from 200.degree. F. to room temperature. This is particularly a problem for thin laminates less than 1/4 inch thick. Polyurethanes capable of laminating at low temperatures have not been used because they tend to easily peel away from polycarbonate and glass.
To overcome this problem of distortion resulting from high lamination temperature, thick laminates are used. For example, Saunders suggests the use of a five layer, laminated glass article which comprises a sheet of polycarbonate, two sheets of polyurethane, and two sheets of glass. However, this laminate is difficult to manufacture, has high material costs, and is unavailable in thin sheets for applications such as 1/4 inch safety glass for jeweler's cases. Another problem with the Saunders laminate is that the glass layer can shatter.
Thus, it would be desirable to have a polyurethane that is capable of lamination at low temperatures, where the polyurethane is optically clear and has sufficient strength for forming strong laminates with glass, polycarbonate, and other materials used in light transmitting laminates.